The Global Positioning System, or GPS, includes a plurality of non-geosynchronous earth-orbiting satellites which transmit signals in the microwave frequency band for reception by earth-based land, sea or air antennas. The various received satellite signals are processed to discern the position of the receiving antenna, generally associated with a vehicle, for general navigational use.
Consistent with the generally accepted objective of providing omni-directional reception capability of the vehicle receiving the GPS signals, the desired maximum gain of the antenna system is substantially zenithal. Such arrangement provides for substantially azimuthally symmetrical reception advantageously allowing for receiving signals from widely separated satellites which signals provide optimal positional resolution.
A preferred antenna for such applications is known as a patch antenna and essentially includes a tuned resonant structure comprising a dual-faced planar ceramic substrate with a thin metallic patch disposed on one face and a grounding conductor disposed on the opposite face. The patch antenna is conventionally utilized in conjunction with an extended ground plane structure which is coupled to the grounding conductor and effective to reduce detrimental external influences on the antenna and maintain the radiation pattern substantially normal to the substrate regardless of surrounding structures. Known varieties of such extended ground planes include package integrated extended ground planes which increase package size of the antenna assembly and external extended ground planes such as the exterior surface of a substantially horizontal vehicle panel. As can be appreciated from the description, an external extended ground plane antenna assembly requires exposed exterior placement on a vehicle which, among other concerns, is aesthetically unacceptable for passenger car applications. A package integrated extended ground plane antenna assembly, while operative autonomously with respect to exterior vehicle panels and substantially unaffected by proximal placement with respect thereto, still suffers from certain trade-offs in the application to passenger car vehicles. For example, optimal azimuthally symmetrical performance dictates that unobstructed roof-top placement be employed. This option, as mentioned, is aesthetically and otherwise unacceptable in passenger car applications. Placement immediately adjacent the interior surface of the windshield or backlight glass also has been proposed but fails acceptance for reasons of (a) visual obstruction from relatively large packaging footprint and (b) substantial signal attenuation in the direction over the vehicle roof panel due to the maximum gain focus being oppositely oriented in accordance with the rake of the glass and direct obstruction of the signal by the roof panel at acute reception angles relative to the horizon. Integration of a package integrated extended ground plane antenna assembly beneath the rear package shelf and vertically below the vehicle backglass has also been explored but also suffers from direct obstruction of the signal by the roof panel.
Patch antennas without extended ground planes have been proposed for glass mount automotive application. Such antennas, however, are generally detrimentally sensitive to proximal placement to the vehicle sheet metal. Hence, proper operation is limited to substantially central placement on the windshield or backglass which is unacceptably within the field of vision of the vehicle operator. Movement closer to the roof panel, hood, or deck lid significantly and detrimentally detunes the antenna from the desired center frequency, changes the gain characteristics and shifts the radiation pattern. Additionally, as previously demonstrated with respect to glass mounted arrangements, substantial signal attenuation in the direction over the vehicle roof panel and direct obstruction of the signal by the roof panel at acute reception angles relative to the horizon remain shortfalls.